LAS VEGAS » Middleweight champion Chris Weidman has a knee injury, forcing the UFC to postpone his title bout against Lyoto Machida for six weeks.

The UFC announced the changes to two major Las Vegas shows on Monday.

Weidman (11-0) was expected to make his second title defense at UFC 173 on May 24 at the MGM Grand Garden, but the Long Island-based mixed martial artist needs minor surgery.

Weidman's bout with Machida is moving to UFC 175 on July 5 at the Mandalay Bay Events Center.

The UFC hasn't decided on a new main event for its traditional Memorial Day weekend show. Former heavyweight champion Junior Dos Santos will meet Stipe Miocic in the top undercard fight on the pay-per-view event.

Weidman claimed the 185-pound title when he stopped long-reigning champion Anderson Silva in the second round last July. He defended the belt in December when Silva broke his leg in their rematch.

Weidman is the fourth champion in the UFC's seven men's weight classes currently sidelined for surgery, although his recovery is expected to be the shortest.

Heavyweight champion Cain Velasquez had shoulder surgery in December, and isn't expected to return to the octagon until late this year. Lightweight champion Anthony Pettis has been out since last August following knee surgery, and is hoping for a late-summer return.

And welterweight champion Johny Hendricks is likely out until fall after tearing his right biceps muscle two weeks before his victory over Robbie Lawler this month.

Machida (21-4), the former UFC light heavyweight champion, got this middleweight title shot after Vitor Belfort was removed from the card in late February when the Nevada Athletic Commission banned the use of testosterone replacement therapy. Belfort was among MMA's most prominent sanctioned steroid users, but has decided to give up the practice.

Machida, a karate expert from Brazil, has won two straight fights after dropping down to 185 pounds.

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